The present invention relates in general to the field of semiconductor silicon wafer image displacement, and more particularly, to a photolithography reticle pattern for use in measuring image displacement and coma aberration of projection optics.
Without limiting the scope of the invention, its background is described in connection with image displacement in semiconductor silicon wafer reticle alignment, as an example.
Heretofore, in this field, alignment marks have been used to position masks for subsequent steps in the processing of semiconductor devices. During the performance of failure analysis, reticle levels and their revisions are generally checked on the semiconductor material. Presently, techniques such as staining in conjunction with scanning electron microscopy and transmission electron microscopy have been used to check the alignment of photolithographic masks after a step that patterns a photoresist. Elemental techniques, such as Auger and secondary ion mass spectroscopy have likewise been used for analyzing semiconductor devices.
Although these techniques are all useful for device failure analysis, it has been found that they provide inconclusive data when used to test for the alignment characteristics of photolithography lenses, such as image displacement and coma aberration and the like. Coma aberration of projection optics in photolithographic patterning systems degrades a resist profile symmetricity and enhance placement error which depends upon pattern size, density and feature.
Several approaches exist which estimate coma aberration of projection optics. Since coma aberration appears in the line-width asymmetry at both ends of several periodic lines, the differences in line-width between both ends are usually measured to characterize the coma aberration. It is, however, difficult to make the mask for this method because both ends of the periodic lines must be the same width as the other. Coma aberration also appears in asymmetric secondary peaks around a contact hole exposed with an attenuated mask. Therefore, the direction of comma aberration is found from the ghost pattern around the contact hole. Furthermore, coma aberration makes relative pattern shifts depending on pattern size and density. This method of measuring the differences in line width between both ends, however, takes a long time for measurement because of the time consuming procedure of using, for example, exposure, Si-etching, exposure with alignment, and measurement using CD-SEM.
Thus, these techniques are time consuming and are ill-suited for use in identifying and characterizing lens and photolithographic misalignment and failure.
In particular, it has been found that present overlay methods will fail to provide accurate information for the critical dimensions used, especially for the future generations of semiconductor devices.
Present image displacement marks are unable to detect image displacement of the projection lense system within the range of present semiconductor device linewidths and pitch sizes, causing large yield drops during semiconductor fabrication.
What is needed, therefore, is a apparatus and method for determining image displacement accuracy following a change in reticle and illumination that is capable of being used with existing stepper optical systems. Also needed is an apparatus and method of rapidly testing optical equipment for proper image displacement performance impacted by image displacement caused by coma aberration at the level of resolution necessary to provide accurate critical dimension readings for very large scale integration (VLSI) and ultra large scale integration (ULSI) devices.
The image displacement test reticle of the present invention may be used to fully characterize the performance of the stepper optical image as well as projection system. More specifically, the image displacement test reticle may be used to solve the displacement discrepancy problems between the current standard image displacement mark and array cells inside the chip, thereby increasing device yield during production.
A reticle, as used herein, is used to describe a photolithographic mask having a test pattern for photolithographic machine evaluation, specifically, coma aberration of projection optics. The test reticle is used for making a mask or reticle alignment and making adjustments to production masks. Mask, on the other hand, is used herein to delineate a production mask used during patterning steps for producing microcircuits.
More particularly, one embodiment of the present invention is directed to a method of measuring coma aberration of the projection optics following a change in illumination including the steps of, taking a first image displacement measurement with a test reticle having a variable linewidth and a variable pitch size on an outer image displacement mark, changing the illumination conditions of photolithographic equipment, such that a variable beam of illumination is projected upon the test reticle and taking a second image displacement measurement with the test reticle to determine an image displacement offset due to changes in illumination.
The present invention may further include changing the illumination conditions of photolithographic equipment projected upon the test reticle and taking a third image displacement measurement. The third measurement may be compared to the first or second image displacement measurement to determine an image displacement offset due to a further change in illumination, such as a different type of illumination than in the first change in illumination. Also, the invention may use a test reticle that further includes an inner box located within the outer image displacement mark. Based on the image displacement offset measured with the test reticle, the image displacement of a reticle, e.g., a production reticle, may be changed during subsequent steps, e.g., printing steps, that involve a change in type of illumination.